This is the staggering logistical nightmare that makes it impossible for the police to keep track of terrorist suspects

Samy Amimour
French police
One of the gunmen who killed 89 people at the Bataclan club in Paris was known to police as a terrorist suspect and was under police monitoring at the time he committed the killings.

Samy Amimour's passport was taken by authorities, and in 2013 the French court system began proceedings against him.

"Materially, physically, you cannot keep watch on 20,000 people round the clock," he says, referring to the rough tally of everyone the French authorities consider to have links to jihadi cells, hold radical Islamist views or pose a threat to national security.

Monitoring one of them for 24 hours requires at least 18-20 agents, who have to be rotated to avoid being recognised. To maintain surveillance over all of them, France would need to deploy 400,000 operatives— more than three times the total number of police officers, gendarmes and soldiers mobilised nationwide in the wake of the attacks.

Ever since Edward Snowden leaked all those NSA documents showing that security services in the UK and US are spying on ordinary citizens en masse, it feels as if the security services must be all-seeing and all-knowing. But as Caprioli's stats indicate, mere knowledge isn't good enough.